The present invention relates to an optical receiver of an optical transmission system for transmitting data using an optical signal.
Recently, the realization of an optical transmission system in which a signal can be transmitted at long distance with S/N ratio (ratio of signal power to noise power) maintained at a low price is desired as optical transmission is popularized. Generally, to transmit at long distance with S/N ratio maintained, the level of a received signal is required to be raised.
FIG. 10 shows the representative configuration of an optical transmission system. An optical transmitter 2 is composed of a multiplexer 33 that multiplexes plural parallel signals, a laser driver 32 and a laser diode 30 that converts an electric signal to an optical signal. An optical receiver 1 is composed of a photodetector 11, a preamplifier 13, an automatic gain controllable (AGC) amplifier 15, a clock (CLK) extraction circuit 16, a decision circuit 17, a demultiplexer 18 and a frequency divider 19. It is parasitic capacitance of an anode terminal 101 of the photodetector 11 that is denoted by a reference number 50 and shown by Cin in the optical receiver 1. The optical transmitter 2 and the optical receiver 1 are connected via an optical fiber 10. The photodetector 11 converts an optical signal received from the optical fiber 10 to a current signal and the preamplifier 13 converts the current signal output by the photodetector 11 to a voltage signal. The AGC amplifier 15 controls the gain so that the amplitude of the output voltage is fixed and the clock extraction circuit 16 extracts a clock from the output of the AGC amplifier 15. The decision circuit 17 discriminates a signal output from the AGC amplifier 15 as a digital signal in synchronization with the clock pulse, the demultiplexer 18 demultiplexes the output of the decision circuit 17 into plural parallel signals in synchronization with a clock pulse output from the frequency divider 19 and these plural parallel signals are sent to a signal processor of a subsequent stage. Also, a current signal may be also converted to a voltage signal using a current detection resistor in place of the preamplifier 13.
In the general optical transmission system described above, to raise the level of a received signal, the light emission power of the laser diode 30 of the optical transmitter 2 has only to be increased, however, generally, there is the relation of tradeoff between the power of the laser diode 30 and high speed, when the light emission power is increased, the frequency characteristic of the optical transmitter is deteriorated and it becomes difficult to realize a flat frequency characteristic in a range of frequencies required for the transmission of a signal.
In the optical receiver 1, to raise the level of a received signal, a large voltage signal can be acquired based upon a small current signal by increasing the ratio (transimpedance) of the output voltage of the preamplifier 13 to a current signal or by increasing the value of the current detection resistor for detecting a current signal and converting it to a voltage signal. For another method, a large current signal can be acquired based upon the small power of light by increasing the area of photodetection of the photodetector 11. As the area of photodetection is increased according to this method, packaging is facilitated and the optical receiver can be manufactured at a low price.
However, as in either method, the parasitic capacitance 50 of the photodetector 11 is caused when the photodetector 11 and the preamplifier 13 or the photodetector 11 and the current detection resistor (not shown) used in place of the preamplifier 13 are connected and a time constant determined by the input resistance of the preamplifier 13 or current detection resistance is increased, the bandwidth of the optical receiver 1 is compressed and a flat frequency characteristic is not acquired in a range of frequencies required for the transmission of a signal. That is, there is a problem that in case means for raising the level of a received signal or facilitating packaging is taken, the flatness of the frequency characteristic of the optical receiver is deteriorated and a sufficient bandwidth cannot be secured.
Conventional type technique for solving such a problem is disclosed in Japanese published unexamined patent application No. Hei 7-135489 for example. It is technique for preventing the flatness of the frequency characteristic of an optical receiver from being deteriorated and securing sufficient bandwidth even if means for raising the level of a received signal or facilitating packaging is adopted by converting current according to an optical signal applied to a photodetector 11 driven at the voltage Vpd of a terminal 102 to voltage by a resistor 14 and compensating parasitic capacitance 50 (Cin) existing in the photodetector 11 using a negative capacitor acquired by a current mirror circuit composed of a capacitor for compensation 43 (C1), an NPN transistor (Q1) and PNP transistors (Q2, Q3) when the converted voltage is applied to a demodulator 20 via the AGC amplifier and an electric signal according to an input optical signal is acquired as shown in FIG. 11. However, the configuration of the circuit disclosed in the technique to concretely realize the negative capacitor has the following problems.
(1) There is the current mirror circuit in which both the NPN transistor (Q1) and the PNP transistors (Q2 and Q3) are used and the PNP transistors (Q2 and Q3) are arranged on a path where the operating speed of the negative capacitor is determined. As well-known, the upper limit of the operating speed of the PNP transistor is lower than that of the NPN transistor and the current mirror circuit is not suitable for high speed operation. Therefore, the upper limit of a frequency for operating the negative capacitor having the circuit configuration shown in FIG. 11 is low. Also, in case high speed PNP transistors are prepared, the increase of the manufacturing cost is a problem.
(2) In the circuit configuration shown in FIG. 11, even if the frequency characteristic of the optical receiver varies because of dispersion in manufacturing the photodetector or circuits and dispersion in parasitic capacitance caused by the packaging of components because the negative capacitor is based upon a fixed value, the variation cannot be compensated.
That is, the optical receiver using the conventional type negative capacitor has problems that the operating speed of the negative capacitor is slow and the frequency characteristic of the optical receiver cannot be optimized by varying a negative capacitance value corresponding to manufacturing dispersion.
The present invention is made to solve the problems. Parasitic capacitance caused in a photodetector and a package is equivalently reduced by configuring a higher-speed variable negative capacitor than the conventional type and the fluctuation of the parasitic capacitance is also compensated. The present invention provides an optical receiver wherein the frequency characteristic is not deteriorated by the variable negative capacitor even if a photodetector having large detection area is adopted, and a current detection resistor having a large resistance value and an amplifier of a first stage to which large resistance is input are used.
According to the invention, difference in quantity between a current signal output from the photodetector and a current signal flowing in a preamplifier or the current detection resistor can be reduced by applying the same AC current as a current signal flowing into one or more parasitic capacitors existing in a process in which a current signal output from the photodetector flows into the preamplifier and the current detection resistor to the preamplifier or the current detection resistor. Also, the loss of a current signal by parasitic capacitance can be reduced by applying the same AC current as a current signal flowing in one or more parasitic capacitors existing in a process in which a current signal flows in a circuit to the parasitic capacitor(s).
Further, even if the parasitic capacitance has dispersion in the manufacturing process of circuits or in the process of packaging components, the quantity of current signals that flow in the preamplifier or the current detection resistor can be regulated because means for regulating the quantity of AC current is provided and the loss of a current signal by parasitic capacitance can be reduced. Hereby, the deterioration of the frequency characteristic of the optical receiver caused because a current signal flows into the parasitic capacitor can be prevented and the optical receiver the frequency characteristic of which is satisfactory can be acquired.